Information/data contained on a record carrier, such as on optical disk, is placed in concentric or spiral tracks surrounding the center of the disk. In order for the information to be written to or read from the disk, the disk write/read assembly moves from one track to another until the track is reached on which information is to be written/read. To accomplish this, each track should be identifiable. And, the write/read assembly should be able to identify tracks as it seeks the location of a track on or from which particular information is to be written or read.
In general, each track is given a unique address which is encoded and written in one or more servo areas on the track. One common coding method involves the conversion of track addresses into gray code words in which only one bit changes between successive words. Such an encoding scheme is less prone to read errors than standard binary code at high disk rotation speeds because a change in only one bit needs to be noted. Different versions of the gray (or creeping) code exist, all of which have the common characteristic that the code words for adjacent tracks differ by only one bit.
Prior to the current invention, however, the use of all such codes has had a common disadvantage. Each bit in the codeword must be recorded on the disk as either a recording mark (a burned hole or pit) or a space and each bit must separately be read as a pit or a space. Consequently, in order to determine whether a bit is a logic zero or a logic one, the read assembly must rely on "threshold detection" techniques, i.e., a comparison is made between the signal reflected from one of the bit positions on the disk and an amplitude reference level. If the reflected signal is greater than the reference level, then the determination is made that the bit is one logic value; if the reflected signal is less than the reference level, then the determination is made that the bit is the other logic value.
A disadvantage of threshold detection is that it assumes that the amplitudes of the reflected signals representing one logic value (such as logic 1) will all be substantially the same and that the amplitudes of the reflected signals representing the other logic value (such as logic 0) will all be substantially the same. However, as the read assembly travels across the tracks, its speed may be such that it will not be positioned exactly over the center of a track during an address read operation. If it is positioned, for example, at one edge of a track the amplitudes of the reflected signals may be less than if the read assembly is positioned closer to the center of the track. Therefore, the reflected signal from a logic 1 may not have sufficient amplitude to exceed the reference level and the bit may be read erroneously as a logic 0.
Furthermore, at high densities, adjacent pits may overlap and they may not be resolved as two distinct pits. Even where a space is used to separate two pits, the pits may encroach onto the space making the space unreadable. Various and complicated coding methods have been developed to overcome this problem (such as requiring that there be no single spaces). However, each bit must still be separately read when threshold detection is utilized thereby leading to the aforementioned drawbacks. If an excessive number of track seek errors are resulted, information access time will increase, information may not be read accurately, and the overall performance of the optical storage system will decrease.
U.S. Pat. No. 4,802,154 discloses a high density code for recording primary data on a record carrier such as an optical disk. In contrast to gray codes, the codes disclosed in the '154 patent use code words in which successive values can differ by more than one bit. Furthermore, each of the two codes disclosed use a fixed number of pits (or holes burned into the disk surface) and can be read using differential detection techniques.
The '154 patent notes several constraints imposed which have the affect of complicating the code as well as complicating the encoding, detection and decoding circuits. For example, the number of pits in even positions equals the number of pits in odd positions to provide a spectral null for synchronization purposes. In addition, there are either no empty positions between adjacent pits or there are two or more empty positions between adjacent pits. This latter constraint is designed to reduce errors which occur when pits which are to be separated by one empty position encroach onto the empty position and conceal it from detection.
Even though each code word contains the same number of pits, each must still be read and decoded as an entire binary word of a particular length. Thus, in one of the embodiments, each code word has four pits and eleven spaces. The detector must determine the location of all four pits out of the fifteen possible positions and then convert the code word into an 8-bit data word. Furthermore, read errors can occur because successive values may differ by more than one pit changing position. Thus, such a method is not well suited to track addressing which requires that the disk read mechanism accurately identify successive tracks as it scans cross them.
The U.S. Pat. No. 4,930,115 discloses a code which can be read with threshold detection and whose code words contain a variable number of pits. Both non-creeping and creeping codes are disclosed. As previously stated, with threshold detection the amplitude of the light reflected from a position is compared with a fixed reference value in order to determine whether the bit at that position is a logic 0 or a logic 1. Errors can occur with threshold detection if the amplitude of the reflection from a space (no pit) is insufficient to exceed the reference level. In such a case, the space would be read as a pit.
Consequently, a need has arisen for an apparatus and method of accurately identifying tracks on a record carrier, such as an optical disk, which does not rely upon threshold detection and/or complicated coding techniques in which the recorded patterns have varying numbers of pits or in which the recorded patterns of successive addresses vary by more than one pit changing position.